1. Field of the Invention
This invention relates to a polyester resin valuable for a toner to be used for the development of statically charged images in an electrophotographic process, an electrostatic recording process, an electro-static printing process and the like. More particularly, it relates to a polyester resin having an excellent blocking resistance, melt flow characteristics and fixing property, and especially valuable for a toner suitable for full-color reproduction where a high melt flowability is required, and for the development in a high-speed copying machine or high-speed printer where a low-temperature fixing property is required, and further for a powder paint.
2. Description of the Related Art
In the process for obtaining a permanent visible image from a statically charged image, the statically charged image formed on a photoconductive photosensitive material or electrostatic recording material is developed with a toner triboelectrically charged in advance and the developed image is fixed. The fixation is performed by directly fusion-bonding the toner image obtained by the development onto the photoconductive photosensitive material or electrostatic recording material, or by transferring the toner image onto a paper or sheet and fusion-bonding the toner image to the transfer sheet. Fusion bonding of the toner image is effected by contact with a vapor of a solvent, pressurization or heating. The heat-fixing method includes a non-contact heating method using an electric oven or flash and a press-heating method using a heating roller. To cope with recent requirements for an increased fixing speed, the latter method is preferably adopted.
In the process for obtaining a color image, toners of three to four colors are applied to a transfer sheet at the above-mentioned developing step, and coloration and fixation are effected while melting and mixing the respective toners at the fixing step. A resin having a good mixing property at the fixing step, that is, a resin having a good melt flowability, is urgently required as the binder for a full-color toner. If a binder having a good melt flowability is used, a problem of an occurrence of an offset phenomenon at the fixing step arises, but if the binder is crosslinked or the molecular weight of the binder is increased, to prevent the offset phenomenon, the melt flowability becomes poor and the binder is not suitable for a full-color toner. Accordingly, a method of preventing the occurrence of an offset phenomenon, which comprises coating a silicone on the surface of a fixing roller of a copying machine, is currently adopted.
At present, an increase of the operation speed in the fixing zone and a realization of energy-saving are urgently required, and in a copying machine other than a full-color copying machine, and a printer, a method is often adopted in which the occurrence of an offset phenomenon is prevented by coating a silicone on a fixing roller.
The use of a low-molecular-weight resin composed of a divalent unsaturated carboxylic acid and an aromatic diol component as a toner resin has been proposed, but since the reactivity of the aromatic diol component with an unsaturated carboxylic acid component is low, to obtain a resin having the desired physical properties, it is necessary to carry out the reaction at a high temperature. If the reaction is carried out at a high temperature, however, the aromatic diol is decomposed and a bad smell is generated and the image quality is poor.
Furthermore, the aromatic diol is relatively expensive, and the use of a toner formed by using a large quantity of the aromatic diol is not preferable from the economical viewpoint during the current popularity of copying machines.
Moreover, since an unsaturated carboxylic acid is used as the divalent carboxylic acid, hydroquinone or the like should be used as an agent for preventing the radical reaction, and the use of this radical reaction-preventing agent can produce environmental pollution.
Under these circumstances, a low-molecular-weight toner suitable for full-color reproduction with a good fixing property, which has a good melt flowability, good heat stability and good polymerization reactivity, is urgently required.